A stent is a medical device used to ameliorate a stenosed or occluded lesion generated in a lumen in vivo such as a blood vessel, a bile duct, a trachea, an esophagus, an urethra, etc. A stent is a netting-like tubular body in which thin struts are joined mutually while intersecting at joints.
Stents are classified, by their function and the manner in which they are indwelled, into two general classifications, self-expandable stents and balloon-expandable stents. A self-expandable stent is a stent which itself has an expanding function, is stored in a catheter in a preliminarily radially contracted state, and is allowed to expand by releasing the contracted state after the stent is positioned in a target lesion so that the stent is fixed in close contact with a lumen. A balloon-expandable stent is a stent which itself does not have an expanding function, but rather is expanded by a balloon positioned in the stent after the stent is positioned in the target lesion, whereby the stent is plastically deformed and fixed in close contact with a lumen.
For instance, in relation to the coronary artery of a heart, a balloon-expandable stent is generally used for the purpose of preventing restenosis after percutaneous transluminal coronary angioplasty (PTCA). In practicing a therapeutic treatment, the stent is mounted onto the outer periphery of a balloon at the distal end of a catheter in a radially contracted state. After insertion into a living body, the stent is expanded (dilated) by the balloon in a target lesion of a lumen and is fixed in close contact with the inner wall of the lumen. The stent is left indwelling in the coronary artery to maintain a dilated state or patency of the stenosed or occluded lesion.
In the case of a stent indwelled in a lumen, the restenosis rate is lower as compared with the case where only PTCA is performed; however, restenosis in the stent indwelling lesion has been observed at a rate of about 20 to 30%. The principal cause of the restenosis is intimal hypertrophy due to migration/growth of blood vessel smooth muscle cells. Recently, therefore, development has been made of stents of the drug eluting type, called DES (Drug Eluting Stents), in which the outer surface of a stent is coated with a drug capable of suppressing the migration/growth of blood vessel smooth muscle cells, and the drug is eluted in the stent indwelling lesion to thereby prevent restenosis.
Examples of the drug include an antibiotic agent such as sirolimus or a carcinostatic agent or the like. Coating with the drug is carried out by a method in which a coating solution prepared by dissolving the drug and a biocompatible polymer in a solvent is applied to the surface of a stent by a dipping method, a spraying method or a so-called direct coating method (applying the solution to the stent along struts which constitute the stent) or the like so that a predetermined quantity of the drug is present on the stent surface, followed by drying to effect solidification.
To put (indwell) a DES in a lumen, however, the stent is radially contracted, is then delivered into the lumen in the contracted state, and is thereafter expanded for indwelling. A problem can thus arise in that the drug coat layer is peeled from the stent surface or tends to be destructed upon expanding/contracting deformation of the stent. In the stents of either type, it is preferable that the stent has flexibility allowing the stent to be deformable not only in the radial direction but also in the axial direction, for coping with a larger number of cases. From this point of view, it is preferable that peeling of the drug coat layer is inhibited or prevented from occurring upon deformation in any direction.
Japanese Patent Laid-Open No. 2007-97706 (for example claim 1 and paragraph No. [0020]) discloses a stent in which a drug coat layer is formed by use of a composition containing a plasticizer. When a plasticizer is thus contained in the drug coat layer, the drug coat layer itself has flexibility and, therefore it is deformed according to expanding/contracting deformations of the stent. This is preferable for preventing the situations of peeling, destruction, damage or falling off of the drug coat layer.
However, a plasticizer is, so to speak, an oil component. Where this substance is put into a lumen in vivo, therefore, it may clog up a peripheral blood vessel if not biodegraded. Even if the plasticizer is biodegraded, it may produce a bad influence on the living body, depending on the type of plasticizer.